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Concept
Definition of planet
Summary
The definition of planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται (), 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.
In modern astronomy, there are two primary conceptions of a 'planet'. Disregarding the often inconsistent technical details, they are whether an astronomical body dynamically dominates its region (that is, whether it controls the fate of other smaller bodies in its vicinity) or whether it is in hydrostatic equilibrium (that is, whether it looks round). These may be characterized as the dynamical dominance definition and the geophysical definition.
The issue of a clear definition for planet came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. In its August 2006 response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter during a meeting in Prague. This definition, which applies only to the Solar System (though exoplanets had been addressed in 2003), states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects approaching its orbit. Pluto fulfills the first two of these criteria, but not the third, and therefore does not qualify as a planet under this formalized definition. The IAU's decision has not resolved all controversies, and while many astronomers have accepted it, some planetary scientists have rejected it outright, proposing a geophysical or similar definition instead.
Geocentric modelHeliocentrismCelestial spheres and Classical planet
While knowledge of the planets predates history and is common to most civilizations, the word planet dates back to ancient Greece.
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Clearing the neighbourhood
"Clearing the neighbourhood" (or dynamical dominance) around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. "Clearing the neighbourhood" is one of three necessary criteria for a celestial body to be considered a planet in the Solar System, according to the definition adopted in 2006 by the International Astronomical Union (IAU).
Planetary science
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their formation. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history.
Planets in astrology
In astrology, planets have a meaning different from the astronomical understanding of what a planet is. Before the age of telescopes, the night sky was thought to consist of two similar components: fixed stars, which remained motionless in relation to each other, and moving objects/"wandering stars" (asteres planetai), which moved relative to the fixed stars over the course of the year(s). To the Ancient Greeks who learned from the Babylonians - the earliest astronomers/astrologers - this group consisted of the five planets visible to the naked eye and excluded Earth, plus the Sun and Moon.